In retrospect, this makes perfect sense. Parental love for children is evolutionarily conditional on the children’s ability to increase the parents’ reproductive success. Stepchildren do not carry any of the genes of the stepparents, so there is absolutely no evolutionary reason for stepparents to love, care for and invest in their stepchildren. Worse yet, any resources invested in stepchildren take away from investment that the stepparents could make in their own genetic children. So, in the cold, heartless calculus of evolutionary logic, it makes perfect sense for the stepfather to kill his stepchildren, so that his mate (the mother of the stepchildren) will only invest in their joint children, children whom the stepfather has had with the mother and who carry his genes. Only they can increase the stepfather’s reproductive success.

But he also cites some contrary evidence from Sweden suggesting that the background characteristics of stepdads do a lot of the work.

In their paper, Temrin et al. do not question that stepchildren are more likely to be killed and maimed by their stepfathers; they only question discriminative parental solicitude as the explanation for it. They point out, and empirically demonstrate with a small Swedish sample, that men who become stepfathers, by marrying women who already have children from previous unions with other men, are more likely to be criminal and violent to begin with. And Temrin et al. argue that their greater tendency toward criminality and violence, not their genetic unrelatedness, is the reason they are more likely to kill and injure their stepchildren.

Once again, in retrospect, this makes perfect sense. Divorced women with children are on average older, so they have lower mate value than younger women without children. Given choice, and all else equal, all men would prefer to marry younger women without children rather than older women with children with other men. The logic of assortative mating would suggest that women with lower mate value are more likely to mate with men with lower mate value. And, as I explain in an earlier post, men with lower mate value are more likely to be criminal and violent.

So a proper New Zealand study would want to correct for the stepfathers’ ex ante characteristics. It would also then partially answer Jan Pryor’s question of the theory, raised in the original Press piece:

The theory also did not explain whether solo-mothers living financially strained lifestyles were targeted by men who preyed upon them and their children, Pryor said.

I’m curious how much of the effect here works through the biological Cinderella story and how much works through the assortative mating dynamics at the lower tail of the distribution.

Add to the list of “open questions that could be answered by somebody with time to muck around in IDI applications and the Stats Data Lab”, or “Masters theses waiting to be written”.

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